In a typical transformer bobbin a non-conductive body supports a plurality of electrical windings, and conductive terminals are embedded in the body. The terminals are connected to respective ends of the windings, and the terminals extend from the body to form external connection terminals (e.g., for insertion in and soldering to a printed circuit board).
In a sectored bobbin, the bobbin typically has a longitudinal form with a hollow interior in which a core member is inserted, and the individual windings are typically wound circumferentially around the bobbin in annular recesses located in sectors spaced therealong. If, as is often desired, the external terminals are at only one end of the bobbin, the windings are typically wound in sequence beginning with that nearest to the terminals and ending with that farthest from the terminals, and the bobbin is typically provided with spacing formations for holding the wire of a farther winding radially outwardly from the nearer winding(s) as the wire extends between the farther winding and the terminals in order to avoid voltage breakdown.
In a layered bobbin, the bobbin typically has a longitudinal form with a hollow interior in which a core member is inserted, and the individual windings are typically wound circumferentially in an annular recess around the bobbin on top of each other at radially spaced positions therearound. If, as is often desired, the external terminals are at only one end of the bobbin and it is desired to begin a winding at one end of the bobbin and to end it at an opposite end, the bobbin is typically provided with a spacing formation for holding the winding wire radially outwardly from the winding as the wire extends between the far end of the winding and the terminals in order to avoid voltage breakdown.
If the bobbin is to support a large number of windings, it may be made of two separate longitudinal portions, one of which fits longitudinally inside the other, and each of which carry windings. The inner bobbin portion must have its terminations at only one thereof so that its other end can fit longitudinally into the outer bobbin portion. In such a bobbin, the use of the spacing arrangement described above to avoid voltage breakdown in the inner portion's windings increases the cross-sectional area of the inner portion and thus increases the size of the outer portion and so increases the size, weight and cost of the resulting transformer. Also in such a bobbin, the use of the spacing arrangement described above to separate the wires of the inner portion's windings increases the air gap between the windings of the inner and outer portions of the core, reducing the magnetic performance of the transformer.